Solid state deformation processing of crosslinked high molecular weight polymeric materials

ABSTRACT

Solid-state deformation processing of crosslinked high molecular weight polymers such as UHMWPE, for example by extrusion below the melt transition, produces materials with a desirable combination of physical and chemical properties. Crosslinked bulk materials are heated to a compression deformable temperature, and pressure is applied to change a transverse dimension of the material. After cooling and stress relieving, a treated bulk material is obtained that has enhanced tensile strength in the axial direction orthogonal to the dimension change. In preferred embodiments, medical implant bearing materials are machined from the treated bulk material with the in vivo load bearing axis substantially parallel or coincident with the axial direction of the treated bulk material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/963,974 filed Oct. 13, 2004 and of U.S. application Ser. No.10/963,975 filed Oct. 13, 2004, and claims the benefit of U.S.Provisional Application No. 60/616,811 filed on Oct. 7, 2004, the entiredisclosures of which are hereby incorporated by reference.

INTRODUCTION

The invention relates to crosslinked high molecular weight polymericmaterial and methods for treating the materials to provide enhancedproperties. In particular, the invention provides methods and materialsfor use in preparing polymeric implants with a high degree of wear andoxidation resistance.

Crosslinked ultra high molecular weight polyethylene (UHMWPE) is nowwidely used in medical implants such as acetabular components for totalhip replacements. There remains interest by the orthopedic community tofind alternative methods of processing radiation crosslinked UHMWPE toimprove mechanical properties while still retaining wear resistance andoxidative stability in the material.

In U.S. Pat. No. 6,168,626, Hyon et al. report enhancement of themechanical properties of crosslinked UHMWPE by deformation processing ata compression deformable temperature. After deformation processing, thematerial is cooled while keeping the deformed state. An oriented UHMWPEmolded article is obtained that has an orientation of crystal planes ina direction parallel to the compression plane. The compression iscarried out using a suitable die or can be done using a hot pressmachine.

Polymeric materials such as UHMWPE can be crosslinked to providematerials with superior wear properties, for example. The polymericmaterials may be chemically crosslinked or preferably crosslinked withirradiation such as gamma irradiation (γ-irradiation). The action ofγ-irradiation on the polymer results in the formation of free radicalswithin the bulk materials. The free radicals provide sites for reactionsto crosslink the molecular chains of the bulk materials. It has becomerecognized that the presence of free radicals, including any freeradicals that survive after subsequent heat treatment, are alsosusceptible to attack by oxygen to form oxidation products. Theformation of such oxidation products generally leads to deterioration ofmechanical properties.

To completely remove free radicals and provide polymeric materials ofhigh oxidative stability, it is known to heat treat the crosslinkedmaterial above the crystalline melting point of the polymer. This has atendency to destroy or recombine all of the free radicals in the bulkmaterial. As a result, the crosslinked material is highly resistant tooxidative degradation. However, some desirable mechanical properties arelost during the melting step.

It would be desirable to provide materials such as crosslinked UHMWPEthat combine a high level of mechanical properties and a high resistanceto oxidative degradation.

SUMMARY

A method of solid state deformation processing of crosslinked polymersincludes deforming a polymer bulk material by compressing it in adirection orthogonal to a main axis of the bulk material and optionallycooling the bulk material while maintaining the deformation force. Whenthe polymeric material is made of UHMWPE and the crosslinking is byirradiation such as γ-irradiation, products of the method areparticularly suitable for use in bearing components and implants fortotal hip replacement and the like.

The level of free radicals in the crosslinked polymer is reduced, butnormally not eliminated, by working the material with the methodsdescribed herein. The bulk material is heated to a compressiondeformable temperature and is then subjected to a force or pressure thatchanges a dimension of the bulk material so that the material flows.Although the disclosure is not limited by theory, it is believed thematerial flow leads to the quenching or reaction of free radicals,leading to a decreased level observed in the solid. Advantageously, thecompression deformable temperature can be chosen below the crystallinemelt temperature of the polymer so that the heat treatment does notadversely affect physical properties. Despite having a measurable(albeit reduced) level of free radicals, the treated bulk material has ahigh degree of oxidative stability, in many cases comparable to bulkmaterial that has been melted to remove free radicals.

In one aspect, the invention involves solid state extrusion of anelongate bulk material while the material is at a compression deformabletemperature, preferably below the melting point. An extrusion dieoperates to apply pressure on the bulk material in a directionorthogonal to the main axis, resulting in compression of the materialand material flow as discussed. The extruded bulk material is thencooled, optionally while held in the deformed state. Alternatively or inaddition, pressure is applied by means of rollers, compression plates,and the like. After cooling, the bulk material is stress relieved byreheating to an annealing temperature to below the melting point, thistime without applying pressure.

An oriented UHMWPE molded article can be obtained according to methodsof the invention by crosslinking a UHMWPE raw article with a high energyray such as gamma-irradiation, heating the crosslinked UHMWPE to acompression deformable temperature, and compression deforming theUHMWPE, followed by cooling and solidifying. Preferably, the rawmaterial is in the form of an object, such as a cylinder or bar,characterized by an axial direction parallel to the main axis of theobject and by a transverse direction orthogonal to the axial direction.The UHMWPE material and the molded article have a detectable level offree radicals, but are resistant to oxidative degradation evidenced by avery low, preferably undetectable, increase in infrared absorption bandsof the UHMWPE material that correspond to formation of carbonyl groupsduring accelerated aging.

In various embodiments, by compression deforming in a directionorthogonal to the main axis of a bulk material, an anisotropic materialis formed wherein mechanical properties in the direction of the mainaxis differ from mechanical properties in the orthogonal or transversedirection. After stress relieving, mechanical properties can differ by20% or more in the axial direction as opposed to the orthogonaldirections. To illustrate, in various embodiments the tensile strengthmeasured in the axial direction of the bulk material is 20% or morehigher than the tensile strength measured in the transverse directions.

Polymers treated by the methods exhibit a desirable combination of hightensile strength and resistance to oxidative degradation. In variousembodiments, transverse deformation of UHMWPE, for example, leads tomaterial having a tensile strength at break greater than 50 Mpa andpreferably greater than 60 Mpa, measured in the axis orthogonal to thedeformation. At the same time, the material is resistant to oxidativedegradation, showing in preferred embodiments essentially no change inoxidation index on accelerated aging.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 illustrates the geometry of an extrusion process;

FIG. 2 shows various embodiments of extrusion apparatus and dies; and

FIG. 3 illustrates a extrusion through a decreasing die.

FIG. 4 shows extrusion through an increasing die.

FIG. 5 illustrates extrusion through an isoareal die.

FIG. 6 illustrates use of sacrificial puck in certain embodiments.

FIGS. 7 and 8 illustrate working the material using rollers.

FIG. 9 shows compression methods of changing the dimensions of thematerial.

It should be noted that the figures set forth herein are intended toexemplify the general characteristics of materials and methods amongthose of this invention, for the purpose of the description of suchembodiments herein. These figures may not precisely reflect thecharacteristics of any given embodiment, and are not necessarilyintended to define or limit specific embodiments within the scope ofthis invention.

DESCRIPTION

The headings (such as “Introduction” and “Summary,”) used herein areintended only for general organization of topics within the disclosureof the invention, and are not intended to limit the disclosure of theinvention or any aspect thereof. In particular, subject matter disclosedin the “Introduction” may include aspects of technology within the scopeof the invention, and may not constitute a recitation of prior art.Subject matter disclosed in the “Summary” is not an exhaustive orcomplete disclosure of the entire scope of the invention or anyembodiments thereof. Similarly, subpart headings in the Description aregiven for convenience of the reader, and are not a representation thatinformation on the topic is to be found exclusively at the heading.

The description and specific examples, while indicating embodiments ofthe invention, are intended for purposes of illustration only and arenot intended to limit the scope of the invention. Moreover, recitationof multiple embodiments having stated features is not intended toexclude other embodiments having additional features, or otherembodiments incorporating different combinations of the stated features.Specific Examples are provided for illustrative purposes of how to make,use and practice the compositions and methods of this invention and,unless explicitly stated otherwise, are not intended to be arepresentation that given embodiments of this invention have, or havenot, been made or tested.

As used herein, the words “preferred” and “preferably” refer toembodiments of the invention that afford certain benefits, under certaincircumstances. However, other embodiments may also be preferred, underthe same or other circumstances. Furthermore, the recitation of one ormore preferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the invention.

As used herein, the word “include,” and its variants, is intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that may also be useful in the materials,compositions, devices, and methods of this invention.

In one embodiment, a method for reducing the free radical concentrationin an irradiated crosslinked bulk polymer is provided. The polymer is inthe form of a bulk material that is elongated in an axial direction.Fixing the axial direction defines transverse directions that areorthogonal to the axial direction. The method involves heating thecrosslinked bulk material to a compression deformable temperature,followed by applying a force to deform the heated bulk material in adirection orthogonal to the axial direction. Thereafter, the polymer iscooled to a solidification temperature. In various embodiments, thepolymer resulting from the processing steps is suitable for furtherprocessing into bearing components for medical implants.

In various embodiments, the polymeric material comprises ultrahighmolecular weight polyethylene (UHMWPE). Methods for processing acrosslinked UHMWPE, wherein the UHMWPE is characterized by having a freeradical concentration greater than 0.06×10¹⁵ per gram, involve heatingthe crosslinked UHMWPE to a compression deformable temperature that ispreferably below the crystalline melting point of the UHMWPE. The UHMWPEis provided in a bulk form characterized by a dimension d₁ in adirectional orthogonal to the axial direction. The method involvesapplying pressure on the UHMWPE in a direction orthogonal to the axialdirection to reduce the dimension to a value d₂ less than d₁.Optionally, the orthogonal pressure is relaxed to permit at least apartial recovery of the dimension d₂ to a dimension d₁′ greater than d₂.Pressure is then optionally reapplied to reduce the dimension to a valued₂′ less than d₁′. In such an embodiment, d₂′ may be less than, equalto, or greater than d₂. The steps of releasing and reapplying orthogonalpressure are repeated in some embodiments to provide a desired amount ofmaterial flow in the bulk UHMWPE. Thereafter, the crosslinked UHMWPE iscooled to a solidification temperature, and a load bearing medicalimplant component is machined from the cooled UHMWPE.Characteristically, the component has a load bearing axis substantiallycoincident with the axial direction of the crosslinked UHMWPE on whichthe pressure steps are carried out.

In another embodiment, a crosslinked UHMWPE is further characterized bycross-sectional area A₁ in a direction orthogonal to the axialdirection. In various embodiments, the crosslinked UHMWPE has aconcentration of free radicals in the bulk material of greater than0.06×10¹⁵ spins per gram. The method involves heating the crosslinkedUHMWPE to a compression deformable temperature that is preferably belowthe crystal melting point. Thereafter, pressure is applied on the UHMWPEbulk material in such a way as to increase the dimension to a value d₂greater than d₁, and at the same time to increase the cross-sectionalare to a value A₂ greater than A₁. Thereafter the pressure is relievedto permit a return to a value d₁′ approximately equal to d₁, and inaerial value A1′ prime approximately equal to A1. The crosslinked UHMWPEis then cooled to a solidification temperature. Optionally, the pressureapplying and pressure relieving steps are repeated as desired to providean amount of material flow in the bulk material. In various embodiments,the method further involves machining a load bearing component from thetreated UHMWPE.

In another embodiment, a crosslinked polymeric material is characterizedby a free radical concentration above the detection limit (0.06×10¹⁵spins per gram or greater) and the bulk material is characterized asabove by an axial direction and further characterized by across-sectional area A1 in a transverse section orthogonal to the axialdirection. The method involves heating the polymeric material to acompression deformable temperature. The heated material is then deformedby extruding it through a die shaped in such a way so as to change thedimensions of the cross-sectional area of the bulk material, but leavethe cross-sectional area essentially unchanged. Thereafter the extrudedmaterial is cooled.

In another embodiment, a method of making a medical implant bearingcomponent is provided. The method comprises

crosslinking a UHMWPE bulk material to produce a free radicalconcentration in the UHMWPE greater than 0.06×10¹⁵ spins per gram;

heating the crosslinked UHMWPE to a compression formable temperature;

extruding the heated UHMWPE, in the form of an elongated bulk materialcomprising a main access defining an axial direction and a cross-sectionperpendicular to the axial direction though a die that has a shapedifferent from that of the cross-section, but having an area essentiallythe same as the cross-section, and further processing the UHMWPE to makethe bearing component.

In various embodiments described herein, extrusion, compression, andother pressure applying steps are carried out once or multiple timesdepending on the desired amount of material “working” and flow desiredin the bulk material. In various embodiments, it is believed that thematerial flow caused by the application of pressure as described hereinis responsible for quenching of free radicals in the polymeric bulkmaterial. As a result, bulk polymeric materials treated according to themethods described herein tend to have lower free radical concentrationsafter the deformation pressure is applied.

In various embodiments, deformation pressure is applied to the bulkmaterial in a direction that is perpendicular to the load bearing axisof a medical implant component later to be machined or otherwiseproduced from the treated bulk material. To illustrate, when thepolymeric bulk material is in the form of a rod, cylinder, or bar,deformation is applied according to the methods described herein in adirection that is orthogonal to the main axis of the polymeric bulkmaterial. The main axis of the cylinder, rod, or bar defines the axialdirection of the bulk material. In various embodiments, deformationpressure is applied by extrusion, rollers, or compression in a directionperpendicular to the axial direction.

In various embodiments, implants are manufactured using preformedpolymeric compositions having the structures described herein and madeby the methods described herein. Non-limiting examples of implantsinclude hip joints, knee joints, ankle joints, elbow joints, shoulderjoints, spine, temporo-mandibular joints, and finger joints. In hipjoints, for example, the preformed polymeric composition can be used tomake the acetabular cup or the insert or liner of the cup. In the kneejoints, the compositions can be made used to make the tibial plateau,the patellar button, and trunnion or other bearing components dependingon the design of the joints. In the ankle joint, the compositions can beused to make the talar surface and other bearing components. In theelbow joint, the compositions can be used to make the radio-numeral orulno-humeral joint and other bearing components. In the shoulder joint,the compositions can be used to make the glenero-humeral articulationand other bearing components. In the spine, intervertebral discreplacements and facet joint replacements may be made from thecompositions.

In various embodiments, the bearing components are made from thepolymeric compositions by known methods such as by machining and areincorporated into implants by conventional means.

Polymers

For implants, preferred polymers include those that are wear resistant,have chemical resistance, resist oxidation, and are compatible withphysiological structures. In various embodiments, the polymers arepolyesters, polymethylmethacrylate, nylons or polyamides,polycarbonates, and polyhydrocarbons such as polyethylene andpolypropylene. High molecular weight and ultra high molecular weightpolymers are preferred in various embodiments. Non-limiting examplesinclude high molecular weight polyethylene, ultra high molecular weightpolyethylene (UHMWPE), and ultra high molecular weight polypropylene. Invarious embodiments, the polymers have molecular ranges from approximatemolecular weight range in the range from about 400,000 to about10,000,000.

UHMWPE is used in joint replacements because it possesses a lowco-efficient of friction, high wear resistance, and compatibility withbody tissue. UHMWPE is available commercially, for example from Ticona,Inc. of Bishop Tex., which sells the GUR series of resins. A number ofgrades are commercially available having molecular weights in thepreferred range described above. The resin is made into bulk materialssuch as bar stock or blocks using various techniques such as compressionmolding or ram extrusion.

In a non-limiting example, the resin is made into a fully consolidatedstock in a series of cold and hot isostatic pressure treatments such asdescribed in England et al., U.S. Pat. No. 5,688,453 and U.S. Pat. No.5,466,530, the disclosures of which are hereby incorporated byreference. The fully consolidated stock is suitable for subsequentcrosslinking and further treatment as described herein.

Crosslinking

According to various embodiments of the invention, a crosslinkedpolymeric bulk material is further processed in a series of heating,deforming, cooling, and machining steps. The polymeric bulk material canbe crosslinked by a variety of chemical and radiation methods.

In various embodiments, chemical crosslinking is accomplished bycombining a polymeric material with a crosslinking chemical andsubjecting the mixture to temperature sufficient to cause crosslinkingto occur. In various embodiments, the chemical crosslinking isaccomplished by molding a polymeric material containing the crosslinkingchemical. The molding temperature is the temperature at which thepolymer is molded. In various embodiments, the molding temperature is ator above the melting temperature of the polymer.

If the crosslinking chemical has a long half-life at the moldingtemperature, it will decompose slowly, and the resulting free radicalscan diffuse in the polymer to form a homogeneous crosslinked network atthe molding temperature. Thus, the molding temperature is alsopreferably high enough to allow the flow of the polymer to occur todistribute or diffuse the crosslinking chemical and the resulting freeradicals to form the homogeneous network. For UHMWPE, a preferredmolding temperature is between about 130° C. and 220° C. with a moldingtime of about 1 to 3 hours. In a non-limiting embodiment, the moldingtemperature and time are 170° C. and 2 hours, respectively.

The crosslinking chemical may be any chemical that decomposes at themolding temperature to form highly reactive intermediates, such as freeradicals, that react with the polymers to form a crosslinked network.Examples of free radical generating chemicals include peroxides,peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, anddivinylbenzene. Examples of azo compounds are: azobis-isobutyronitrile,azobis-isobutyronitrile, and dimethylazodiisobutyrate. Examples ofperesters are t-butyl peracetate and t-butyl perbenzoate.

Preferably the polymer is crosslinked by treating it with an organicperoxide. Suitable peroxides include2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne (Lupersol 130, AtochemInc., Philadelphia, Pa.); 2,5-dimethyl-2,5-di-(t-butylperoxy)-hexane;t-butyl α-cumyl peroxide; di-butyl peroxide; t-butyl hydroperoxide;benzoyl peroxide; dichlorobenzoyl peroxide; dicumyl peroxide;di-tertiary butyl peroxide; 2,5-dimethyl-2,5-di(peroxybenzoate)hexyne-3; 1,3-bis(t-butyl peroxy isopropyl) benzene; lauroylperoxide; di-t-amyl peroxide; 1,1-di-(t-butylperoxy)cyclohexane;2,2-di-(t-butylperoxy)butane; and 2,2-di-(t-amylperoxy) propane. Apreferred peroxide is 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne.The preferred peroxides have a half-life of between 2 minutes to 1 hour;and more preferably, the half-life is between 5 minutes to 50 minutes atthe molding temperature.

Generally, between 0.2 to 5.0 wt % of peroxide is used; more preferably,the range is between 0.5 to 3.0 wt % of peroxide; and most preferably,the range is between 0.6 to 2 wt %.

The peroxide can be dissolved in an inert solvent before being added tothe polymer powder. The inert solvent preferably evaporates before thepolymer is molded. Examples of such inert solvents are alcohol andacetone.

For convenience, the reaction between the polymer and the crosslinkingchemical, such as peroxide, can generally be carried out at moldingpressures. Generally, the reactants are incubated at moldingtemperature, between 1 to 3 hours, and more preferably, for about 2hours.

The reaction mixture is preferably slowly heated to achieve the moldingtemperature. After the incubation period, the crosslinked polymer ispreferably slowly cooled down to room temperature. For example, thepolymer may be left at room temperature and allowed to cool on its own.Slow cooling allows the formation of a stable crystalline structure.

The reaction parameters for crosslinking polymers with peroxide, and thechoices of peroxides, can be determined by one skilled in the art. Forexample, a wide variety of peroxides are available for reaction withpolyolefins, and investigations of their relative efficiencies have beenreported. Differences in decomposition rates are perhaps the main factorin selecting a particular peroxide for an intended application.

In various embodiments, crosslinking is accomplished by exposing apolymeric bulk material to irradiation. Non-limiting examples ofirradiation for crosslinking the polymers include electron beam, x-ray,and γ-irradiation. In various embodiments, γ-irradiation is preferredbecause the radiation readily penetrates the bulk material. Electronbeams can also be used to irradiate the bulk material. With e-beamradiation, the penetration depth depends on the energy of the electronbeam, as is well known in the art.

For γ-irradiation, the polymeric bulk material is irradiated in a solidstate at a dose of about 0.01 to 100 MRad (0.1 to 1000 kGy), preferablyfrom 0.01 to 10 MRad, using methods known in the art, such as exposureto gamma emissions from an isotope such as ⁶⁰Co. In various embodiments,γ-irradiation is carried out at a dose of 0.01 to 6, preferably about1.5 to 6 MRad. In a non-limiting embodiment, irradiation is to a dose ofapproximately 5 MRad.

Irradiation of the polymeric bulk material is usually accomplished in aninert atmosphere or vacuum. For example, the polymeric bulk material maybe packaged in an oxygen impermeable package during the irradiationstep. Inert gases, such as nitrogen, argon, and helium may also be used.When vacuum is used, the packaged material may be subjected to one ormore cycles of flushing with an inert gas and applying the vacuum toeliminate oxygen from the package. Examples of package materials includemetal foil pouches such as aluminum or Mylar® coating packaging foil,which are available commercially for heat sealed vacuum packaging.Irradiating the polymeric bulk material in an inert atmosphere reducesthe effect of oxidation and the accompanying chain scission reactionsthat can occur during irradiation. Oxidation caused by oxygen present inthe atmosphere present in the irradiation is generally limited to thesurface of the polymeric material. In general, low levels of surfaceoxidation can be tolerated, as the oxidized surface can be removedduring subsequent machining.

Irradiation such as γ-irradiation can be carried out on polymericmaterial at specialized installations possessing suitable irradiationequipment. When the irradiation is carried out at a location other thanthe one in which the further heating, compressing, cooling, andmachining operations are to be carried out, the irradiated bulk materialis conveniently left in the oxygen impermeable packaging during shipmentto the site for further operations.

Bulk Form of the Materials

The crosslinked polymer is provided in a bulk form characterized by anaxial direction and a transverse direction orthogonal or perpendicularto the axial direction. In subsequent processing steps, pressure isapplied to change a dimension in a transverse direction on thecrosslinked bulk material to “work” the material and induce at leastsome material flow in response to the pressure. In various embodiments,the axial direction corresponds to an elongated dimension or directionof the bulk material, when the material is provided for example in theshape of a cylinder of rod of circular or other cross section. The axialdirection is essentially parallel to or coincident with the main axis ofthe material. In other embodiments, the axial direction does notnecessarily correspond visually to an elongated direction, but rather isdefined in relation to the direction in which enhanced physicalproperties such as tensile strength are developed. In preferredembodiments, for example, load bearing components are produced from thetreated bulk material in an orientation where the load bearing axiscorresponds approximately or exactly to the axial direction.

For convenience of reference, a set of 3-D orthogonal axes is set upwith one of the axes coincident with the axial direction. To illustrate,the “z” axis can be taken as coincident with the main cylindrical axisof bulk material in the form of a cylinder. The other two axes lie in aplane perpendicular to the axial direction. Thus the “x” and “y” axeslie perpendicular to the axial direction, and represent transverse axesor transverse directions in the bulk material. The orientation of “x and“y” axes is arbitrary in the transverse plane. When the cross section ofthe bulk material is circular, all possible orientations of the “x” and“y” transverse axes are equivalent. With other shapes, “x” and “y”components of the bulk material depend on the arbitrarily chosen axes.But in all cases, the shape of the transverse cross section of the bulkmaterial, and the changes in dimension upon application of pressurealong those axes, can be expressed as a combination on the basis of the“x” and “y” axes. In various embodiments, a change in dimension inresponse to an applied pressure occurs in the direction of the appliedpressure, which direction can be expressed algebraically and vectorallyas a combination of “x” and “y” components.

In various embodiments, the axial direction is constant throughout thebulk material. This is the case for, say, straight cylinders where the(straight) main axis of the cylinder is taken as the axial direction.This is a preferred arrangement. But the bulk material can also beprovided in the form of an elongated body where the main axis changesdirection along the axial direction. Such would be the case for bent orcurved rods, for tori, and for other closed shapes, by way ofnon-limiting example. For such bulk materials, the transverse directionsare still defined as those directions which are at right angles to theaxial direction, whatever the local orientation of the axial directionof the bulk material. The methods described herein are readily adaptedto provide changes in dimensions in the bulk material at right angles tothe (local) axial direction.

The shape of the cross section transverse to the axial direction of thebulk material is not particular limited and includes circles and theirtopological equivalents (such as ovals, ovoids, ellipses, and otherareas bounded by a closed curve), as well as other shapes. Non-limitingexamples of shapes include regular and non-regular polygons (e.g.squares, rectangles, rhombus, and trapezoids for four sided figures),stars, convex shapes, and concave shapes. Certain shapes are preferredbecause of their relative ease of manufacture. Such include circularcross-sections, readily produced by RAM extrusion for example.

The axial direction is the direction in which higher tensile strength isdeveloped, as described further below. In this aspect, the axialdirection of the bulk material is the direction perpendicular to thedimensional change in the transverse direction that results fromapplication of pressure. In various embodiments, application of pressureor force orthogonal to the axial direction creates an anisotropicmaterial, characterized by higher tensile strength in the axial than inthe transverse direction.

The axial direction of the bulk material also defines the preferreddirection in which implant bearing components such as acetabular cupsare to be machined. That is, bearing components are preferably made ormachined from the treated bulk polymer in an orientation where thehigher tensile strength axis of the polymeric bulk material correspondsto the load bearing axis or direction of the bearing component of theimplant in vivo.

In an exemplary embodiment, the bulk material is in the form of a rod orcylinder having a circular cross section. The axial direction isparallel to the main axis of the cylinder, while the transversedirections are at right angles to the axial direction. In other words,the existence of the axial direction defines an orthogonal directionreferred to as “transverse” in this application. When the cross sectionof the bulk material is isotropic as in the case of a cylinder, thetransverse direction can be described as “radial”, and the transverseaxis as a radial axis. The main axis of the bulk material can also becalled the longitudinal axis. As used here, the longitudinal axis isparallel to the axial direction.

In the non-limiting case of a rod or cylinder, a cross section of thebulk material perpendicular to the axial direction or longitudinal axisis a circle. Other bulk materials characterized by an axial directionmay be used that have other perpendicular cross sections. In anon-limiting example, a square cylinder can be provided that has asquare cross section perpendicular to the axial direction. Other bulkmaterials characterized by an axial direction can have rectangular,polygonal, star, lobed, and other cross sections perpendicular to theaxial direction.

In various embodiments, the axial direction of the bulk polymericmaterial is elongated compared to the orthogonal or radial direction.For example, in the case of UHMWPE, a commercially available bulkmaterial is a cylinder approximately 3 inches in diameter and 14 inchesin length. The length corresponds to the axial direction and thediameter corresponds to the radial direction. As described below,bearing components for implants are preferably machined from billets cutin the axial direction. For efficiency in manufacturing it is convenientto produce a number of bearing components from a single bulk materialtreated by the methods of the invention. For this reason, the bulkmaterial is usually to be extended in an axial direction so as to beable to cut a plurality of billets from the material for use in furthermachining of the bearing components.

As described above, bulk material characterized by an axial direction isfurther characterized as having a variety of cross sectional areasperpendicular to the axial direction. In various embodiments, thedimensions of the cross sectional areas perpendicular to the axialdirection are more or less constant along the axial direction from thebeginning to the end or from the top to the bottom of the bulk material.In various other embodiments, bulk materials may be provided to havecross sectional areas that vary along the length or axial direction ofthe bulk material. In the case where the cross sectional area of thebulk material is constant along the axial direction of the bulkmaterial, compressive force applied as described below will generally beapplied to the bulk material in a direction perpendicular to the axialdirection. In the case where the cross sectional area varies along theaxial direction of the bulk material, compressive force applied to thebulk material may have a component in the axial direction due to thegeometry of the bulk material. However, in all cases at least acomponent of the compressive force will be applied on the bulk materialin a direction orthogonal to the axial direction.

Pre-heating

Before further processing, the crosslinked polymer is heated to acompression deformable temperature. The compression deformabletemperature is temperature at which the polymeric bulk material softensand can flow under the application of a compressive source to changedimension in the direction the compressive force is applied. For UHMWPEand other polymeric materials, the compression deformable temperature isconcretely from about the melting point minus 50° C. to the meltingpoint plus 80° C.

In various embodiments, the compression deformable temperature is belowthe melting point of the polymeric material. Examples of the compressiondeformable temperature include from the melting point to 10° C. belowthe melting point, from the melting point to 20° C. below the meltingpoint, from the melting point to 30° C. below the melting point, andfrom the melting point to 40° C. below the melting point. For UHMWPE,the compression deformable temperature is preferably above 80° C., orfrom about 86° C. to about 136° C., since the melting temperature of theUHMWPE is about 136° C. to 139° C. In various embodiments, thecompression deformable temperature of UHMWPE lies from about 90° C. to135° C., preferably about 100° C. to 130° C. A preferred temperature is125-135° C., or 130° C.±5° C.

In various embodiments, the crosslinked material is heated to acompression deformable temperature above the melting point of thepolymer. For UHMWPE and other polymeric materials, such a compressiondeformable temperature is from just above the melting point to atemperature about 80° C. higher than the melting point. For example,UHMWPE can be heated to a temperature of 160° C. to 220° C. or 180° C.to 200° C.

In various embodiments, it is preferred to heat the bulk polymericmaterial to a compression deformation temperature close to but nothigher than the melting point. In various embodiments, the compressiondeformable temperature is between the melting point and a temperature20° C. lower than the melting point, or between the melting point and atemperature 10° C. lower than the melting point.

The crosslinked bulk material can be heated to a compression deformabletemperature in a deformation chamber as illustrated in the figures, orit can be preheated in an oven to the compression deformabletemperature. In various embodiments, the bulk material is heated to atemperature just below the melting point, such as the melting pointminus 5° or the melting point minus 10° and placed in a heateddeformation chamber. The deformation chamber preferably maintains acompression deformable temperature. If desired, the deformation chambercan be heated or thermostatted to maintain a constant temperature.Alternatively, the deformation chamber is not itself heated but hassufficient insulating properties to maintain the bulk material at acompression deformable temperature during the course of pressureapplication described below. In various embodiments, the temperature ofthe deformation chamber is held at several degrees below the meltingtemperature to avoid melting.

Deformation

When the crosslinked bulk material is at a compression deformabletemperature, pressure is applied to the bulk material to induce adimensional change in a direction orthogonal to the axial direction. Thedimensions of the bulk material change in response to the application ofpressure, which results in “working” of the crosslinked material withmaterial flow of the heated bulk material. Force (or, equivalently,pressure, which is force divided by area) is applied so that least onecomponent of the dimension change is orthogonal to the axial directionof the bulk material, with the dimensional change being either positiveor negative. To illustrate, for cylindrical rods and other bulkmaterials that have a constant cross section along the axial directionof the bulk material, compression force is applied in a directionperpendicular to the axial direction in order to decrease an axialdimension

Any suitable methods may be used to apply the compression force in adirection orthogonal to the axial direction. Non-limiting examplesinclude extrusion through dies and the use of rollers, compressionplates, clamps, and equivalent means.

Extrusion

In various embodiments, deforming force is applied in the directionorthogonal to the axial direction of the bulk material by extruding thebulk material through a die or series of dies. Extrusion through the dieor dies is carried out in the axial direction of the bulk material.Suitable dies can be manufactured by traditional machining methods, byelectrical discharge machining, or by other techniques well known in themachine tool art.

A die used in the extrusion processes has a shape and size selected toapply pressure in a transverse direction to the bulk material in thedesired fashion. Examples of shapes include, without limitation, circlesand their topological equivalents, shapes bounded by closed curves,shapes bounded by a closed curve made up of straight segments and curvedsegments, regular polygons, and non-regular polygons. Withoutlimitation, the die has one or more of concave shapes, convex shapes,smooth shapes (for example, having a continuous derivative), smoothshape with a kink or kinks (for example, points where the derivative isnon-continuous), or shapes bounded by straight lines (e.g. regular andnon-regular polygons). In various embodiments, certain die shapes arepreferred for their ease of manufacture. The choice of die shape is alsoto be made in consideration of compatibility with the shape and size ofthe bulk material to be worked.

The act of extruding converts the pressure of an extrusion ram in anaxial direction to pressure in the transverse direction applied by thedie. Depending on the configuration of the die used, and thecross-sectional shape and area of the workpiece in relation to thecross-sectional shape and area of the die, extrusion is accomplishedwith a reduction in cross-sectional area (a “reducing die”), with anincrease in cross-sectional area (an “increasing die”) or with no changein cross-sectional area (an “isoareal die”). Various extrusionembodiments are illustrated in the Figures. It is to be noted that theFigures are not necessarily drawn to scale and do not necessarilyrepresent preferred configurations for any individual embodiment; theFigures are used to illustrate the concepts.

Reducing Die

A reducing die has a cross-sectional area that is less than thecross-sectional area of the piece to be extruded through the die. Duringextrusion through a reducing die, pressure exerted on the bulk materialin a direction orthogonal to the axial direction causes at least onetransverse dimension of the bulk material to be reduced compared to theoriginal dimension of the bulk material. In other words, the diameter orother transverse dimension of the bulk material after extrusion is lessthan the dimension before extrusion.

The relative reduction in the dimension of the bulk material in thetransverse directions can be expressed as a ratio of the originaldimension d₁ to the reduced dimension d₂. Depending on the method ofreducing the dimension by applying compressive force, the numeric valueof the ratio d₁/d₂ can be referred to as a draw ratio or a diametralcompression. For extrusion, it is common practice to refer to a drawratio; unless stated otherwise from context, the term draw ratio will beused here to refer to all geometries.

It is to be understood that the transverse direction (the directionorthogonal to the axial direction) itself contains two axes that can bedrawn at right angles to the longitudinal axis. In various embodiments,the bulk material can be deformed by a different amount along the twotransverse axes, and a draw ratio can be defined for both axes (or,equivalently, any linear combination of the two axes). The orientationof the transverse axes is arbitrary; if needed for analysis, the axescan be selected to simplify the geometry of the applied forces. When thecross section of the bulk material is circular, equal deformation forcecan be applied in all transverse directions by extruding through acircular reducing die. In this non-limiting case, the dimension d₂corresponds to the radius or diameter of the extruded material, and thedraw ratio is the fraction defined by dividing d₁ by d₂.

In various embodiments, the draw ratio is 1.1 or higher, and less thanabout 3. In various embodiments, the draw ratio is 1.2 or higher, and ispreferably about 1.2 to 1.8. It is about 1.5 in a non-limiting example.Minimum levels of draw ratio provide sufficient material flow in thecrosslinked material to provide benefits described herein. As thecrosslinked material is “worked” to a greater extent, the dimensions inthe transverse direction change to a greater extent, thus increasing thecalculated draw ratio. As the draw ratio is increased, a point iseventually reached at which the strain introduced by the dimensionchange is too great and the properties of the crosslinked polymericmaterials deteriorate. Accordingly, in various embodiments the drawratio is 3.0 or less, 2.5 or less, and preferably about 2.0 or less. Ina preferred embodiment, the compressive force is applied more or lessisotropically around the bulk material in a direction transverse to alongitudinal axis. Accordingly, the reduction in dimension will usuallyapply in all transverse directions. To illustrate, a circular crosssection remains round but is reduced in diameter, while a polygonalcross section such as a square or rectangle is reduced on all sides.

When compressive force is applied anisotropically with a reducing die,the two dimensions of the workpiece orthogonal to the long axialdirection change by different amounts. At least one is reduced to suchan extent that the cross-sectional area of the die is less than that ofthe workpiece. Examples of dies that apply force or pressureanisotropically include ovals, rectangles, and other “asymmetric”shapes. Asymmetric shapes include those resulting from deformation of acircle, such as ellipses, ovals, and the like.

The geometry of extrusion through a reducing die is illustrated inschematic form in FIGS. 1 and 2. A reducing die 6 is disposed between adeformation chamber 2 and a holding chamber 4. As shown, the holdingchamber has the same diameter as the die. In various embodiments it isequipped with water or other cooling means to slowly cool the extrudedmaterial from its compression deformable temperature. The figure isshown in a cross sectional view to illustrate that the reducing die 6reduces the diameter or dimension of the extruded material from anoriginal dimension d₁ to an extruded dimension d₂, as extrusion is fromleft to right in the Figure. As the crosslinked heated bulk materialpasses from the deformation chamber through the reducing die 6, thematerial flows by the die wall 5 that leads to a constriction 10 havingthe diameter d₂ of the cooling chamber 4.

Various geometries of the reducing die are illustrated in non-limitingform in FIG. 2. FIGS. 2 a to 2 e show the relative configuration of thedeformation chamber wall 20 and the die constriction 10. The die wall 5is seen to connect the cooling deformation chamber to the coolingchamber. In FIG. 2 a, the cross section of both the deformation chamber2 and cooling chamber 4 are circular (indicated with the illustration ofthe relative disposition of the chamber wall 20 and the restriction 10),with dimensions d₁ and d₂ corresponding to their respective diameters.In FIG. 2 b, the deformation chamber is square or rectangularcharacterized by a dimension d₁ that can be arbitrarily taken along adiagonal or along a side. In FIG. 2 b, the restriction 10 is alsorectangular but having lower dimension d₂. FIGS. 2 c through 2 eillustrate other combinations of circular, square, and triangulardeformations and cooling chambers connected by reducing dies 6 having adie wall 5, and are offered by way of non-limiting example.

As noted above, the bulk material in the deformation chamber 2 is heldat a compression deformable temperature. At such a temperature, thematerial can flow in response to pressure exerted on the material. Whenthe compression deformable temperature is below the melting point, thematerial undergoes a solid state flow through the reducing die 6.Pressure or force applied to the end of the bar by the ram is translatedby the die into compressive force that reduces the dimension of the bulkmaterial in the transverse direction. For convenience, the deformationchamber illustrated in FIG. 1 can be sized to match relatively closelythe diameter or dimension d₁ of the bulk material to be extruded.

When the material is extruded into a holding chamber as illustrated inFIG. 1, as noted the material can be cooled down while being held in adeformed state to the extent that d₂ is less than d₁. Alternatively, thematerial can extruded through the die into a region of ambient pressure,allowing the bulk material to immediately recover from the dimensionchange from d₁ to d₂. When not held at a reduced dimension, the extrudedcrosslinked material tends to return at least somewhat to its originalshape, so that d₂ tends to recover approximately to its originaldimension d₁. Normally, the recovery occurs within seconds of beingextruded through the die into ambient pressure conditions.

Extruding the bulk material at draw ratios of about 1.1 or higher asdescribed above works the extruded material by inducing material flow asthe dimensions are reduced by passing through the extrusion die. Invarious embodiments, the amount of the working is determined by therelative draw ratio. Thus, preferred draw ratios are 1.1 or higher andless than about 3. In some embodiments, it is desired to increase theworking of the material without causing too high a strain on the part,for example by extruding at a high draw ratio.

One way of increasing the working of the material without overlyincreasing the draw ratio is to carry out a sequential extrusion of thebulk material through a die. In a simple embodiment, the method involvesextruding the bulk material through the reducing die, collecting thematerial as it comes out the die, and providing the material as input toanother die or the same die. In this way, the material can bere-extruded to provide a desired amount of working, which leads toreduction in the level of free radicals in the material. Non-limitingembodiments of the sequential extrusion are illustrated in FIG. 3.

FIG. 3 a shows extrusion of a bulk material 50 through a constriction 10in a die that reduces the dimension from d₁ to d₂ in the transversedirection. Upon passing out of the die, the pressure applied by theconstriction is relieved (by passing into a region of ambient pressure)and the dimension of the bulk material tends to increase to a dimensiond₁′, which is generally the same as or slightly less than the originaldimension d₁. If it is desired to work the bulk material further, thebulk material 50 can be re-extruded through the die shown in FIG. 3 a.In the second extrusion, a bulk material of dimension d₁′ is extrudedthrough the reducing die. After emerging from the extruding die, thediameter is given as d₁″, which as before tends to be about the same orslightly less than d₁′. In this way, an arbitrary number of extrusionscan be carried out on a bulk material 50 to provide a desired (reduced)level of free radicals in the bulk material, the lower level arisingfrom the working of the material by extruding through the die.

An alternative embodiment is shown in FIG. 3 b. Here, the bulk material50 is extruded through a constriction 10 into a chamber 15 having adimension d₂ of less than the original dimension d₁. However, in FIG. 3b, the extrusion apparatus is sized so that the bulk material has ahigher volume than chamber 15. As a result, during extrusion the bulkmaterial is extruded out of the apparatus, whereby it regains dimensionto a value d₁′ that, as before, is approximately the same as or slightlyless than the original d₁. The worked materials 50 can be re-extruded asbefore.

The examples shown in 3 a and 3 b are characterized as multipleextrusions through a single die. Working can also be provided byextruding through a series of dies as illustrated in non-limitingfashion in FIGS. 3 c and 3 d. FIG. 3 c shows a series of dies that takethe diameter of the bulk material from an original diameter d₁ to afirst constricted diameter d₂ to a second expanded d₁′ to a secondrestricted diameter d₂′ and finally to a final dimension d₁″. In variousembodiments, it is contemplated to extrude the bulk material (not shown)from the series of dies to ambient conditions where the diameter returnsapproximately to the original diameter d₁. Another embodiment is shownin non-limiting fashion in FIG. 3 d, where a series of dies is set up totake bulk material from an original dimension d₁ to a second dimensiond₂ to a third dimension d₃. As before, it is contemplated to extrude thebulk material through the apparatus, whereby the dimension recovers to avalue close to or slightly less than the original dimension d₁. Ifdesired, material that has been passed through a series of dies as shownin non-limiting fashion in FIGS. 3 c and 3 d can be re-extruded throughthe same apparatus or through apparatus such as shown in FIGS. 3 a and 3b. In all cases, re-extrusion works the material further, whichgenerally leads to a reduction in concentration of free radicals, forexample in the crystalline regions of the bulk material.

As with the single dies illustrated in FIGS. 1 and 2, in variousembodiments the material is extruded through a series of dies asillustrated in FIG. 3, but into a holding chamber or other device tomaintain the dimension of the extruded material at a reduced valuecompared to the original dimension d₁. The material is then cooled whilemaintaining the compressed state.

Increasing Die

Another way of working the crosslinked material to reduce the level offree radicals is to induce flow in the bulk material by extruding it inan increasing die. When the temperature is below the crystalline meltingpoint of the UHMWPE or other crosslinked material, the flow in the bulkmaterial is characterized as solid state flow. The method is illustratedin non-limiting fashion in FIG. 4. In FIG. 4 a, shown in cross section,the bulk material 50 is extruded through a restriction 10 into an areaof increasing volume defined by the walls 5 to a chamber 15 that ischaracterized by a dimension d₂ that is greater than dimension d₁. Asillustrated, it is normally desirable to provide at least a slightamount of back pressure 54 to cause the bulk material to flow outward tofill the chamber 15. Back pressure is provided by any suitable means,such as without limitation with a ram, with a sacrificial puck set up asdescribed below, or with fluid pressure. Upon removal of the pressure54, for example by extruding it out of the extrusion apparatus, thedimension d₂ tends to return to a value close to the original dimensiond₁. As drawn in FIG. 4, the extrusion apparatus further comprises adecreasing wall 5′ leading to a second constriction 10′ that defines areducing die as illustrated for example in FIG. 3. In a preferredembodiment, after extruding through an increasing die as shown in FIG. 4a and into the chamber 15 (FIG. 4 b), the bulk material is furtherextruded through a decreasing die as further illustrated in FIG. 4 c tobring the dimension back to a value d₁′ which is less than d₂.Alternatively or in addition, the bulk material is extruded through aseries of increasing dies analogous to that shown in FIG. 3 d for thedecreasing die. In various embodiments, the bulk material 50 is workedby extruding through a sequence of increasing and decreasing dies asillustrated schematically in FIG. 4.

Isoareal Die

In a particular embodiment, the bulk material is worked by extrudingthrough a die that changes the dimensions of the cross section of theelongated bulk material being extruded, but does not substantiallychange the cross sectional area. Working of the bulk material isprovided by extruding, although no net change in area occurs. In variousembodiments, the isoareal extrusion is advantageous because theextrusion requires less ramming force, since there is less resistance toflow because of the small or zero value net area change in the crosssection.

Thinking in terms of the x and y axis of the cross sectional area, it isclear that for an isoareal extrusion through a restriction, it isnecessary that least one dimension of the cross section decrease whileanother increase. The die itself can be conceptualized as providing afirst cross sectional area, which in most practical applications issubstantially the same as the cross section of the bulk material to beworked. The isoareal die also provides a second cross sectional areawhich has the same cross sectional area, but has different dimensions.In a simple illustration the first shape is a circle and the secondshape is an oval or ellipse having the same area as the circle. Thedimension of the circle is conveniently given by a radius or diameter,while the dimensions of the oval are provided by a major and minor axis.The values of diameter, and a major and minor axes are selected to giveisoareal circles and ovals according to well known geometricalprinciples. Of course, the method can be generalized to provide isoarealshapes for any arbitrary starting configuration of the bulk material. Asnoted, a common bulk form of crosslinked UHMWPE is a cylinder ofapproximately 3 inches in diameter and 14 inches in length. Accordingly,isoareal extrusion is provided for the exemplary crosslinked bulkmaterial if the second shape has a cross sectional area of approximately7.06 square inches (πd²/4 when the diameter d is 3 inches).

As a physical practicality, there is a transition zone between the firstdie shape and the second die shape. In preferred embodiments, the crosssectional area in this transition zone is the same as the first shapeand the second shape, as it “morphs” from the first shape to the secondshape to provide isoareal restriction. In various embodiments, theabsolute requirement of mathematical precision in the machining of suchdie shapes and transitions zones is relaxed slightly; the key feature ofthis aspect is that, due to the approximately equal areas of the twoshapes, extrusion through the die takes place with a minimum of powerinput to provide advantages as described herein.

The geometry and set up of isoareal dies is illustrated in non-limitingfashion in FIG. 5. FIG. 5 a shows an extrusion apparatus, where 20 isthe wall of an elliptical die tool. The extrusion apparatus isillustrated showing isoareal extrusion shapes indicated as 200, 300,400, 500, and 600. The shapes are connected by transition zones havinginner walls 21, 22, 23, and 24. A view along cut line 5 b illustratesthat in relation to circular shape 200, shape 300 is a vertical oval.The view along 5 c shows that shape 400 goes back to a circle. The viewalong line 5 d shows that isoareal shape 500 is an oval turned at 90° tothe oval of shape 300. The view along line 5 e shows shape 600 is againa circle. Working of a crosslinked bulk material is accomplished asbefore by pushing a bulk material (not shown) through a die or series ofdies illustrated as 200, 300, 400, 500, and 600 in FIG. 5. As before,the bulk material can be further worked by subsequent re-extrusionthrough the same apparatus. As illustrated with isoareal extrusion, invarious embodiments it is preferred to provide a plurality of isoarealrestriction steps, including changes in shape along different axes. Thisprovides material flow in more than one dimension and tends to lead tobetter free radical concentration reduction induced by the materialflow.

To further illustrate, top and side views, respectively, of the righthand side of FIG. 5 a are given in FIGS. 5 f and 5 g. They show the ovalisoareal shape 500 disposed between two circular die shapes 400 and 600.

Sacrificial Puck

In a preferred embodiment, a so-called sacrificial puck is used toimprove the efficiency of the extrusion process. The concept of using asacrificial puck is illustrated in FIG. 6; its use is readily adapted toother configurations. In referring to FIG. 6, a ram 30 is provided in aretracted position with respect to the deformation chamber 2. FIG. 3 bshows the ram 30 retracted and the deformation chamber 2 filled with arod-like bulk material 50 and a sacrificial puck 40. The sacrificialpuck 40 is preferably made of a crosslinked polymer, which may be thesame as the crosslinked polymer of the bulk material 50. It ispreferably of approximately the same cross-sectional shape and area asthe bulk material 50 to be extruded. In FIG. 3 c, the ram 30 is shownpushing on the sacrificial puck 40, which in turn pushes on the bulkmaterial 50 to move the bulk material 50 through the reducing die 6 intothe cooling chamber 4. FIG. 3 d shows the situation at the end of thestroke of the ram 30. The bulk material 50 is sitting completely in thecooling chamber 4, while the sacrificial puck 30 occupies the reducingdie 6. Upon retraction of the ram 30 as shown in FIG. 3 e, thesacrificial puck 40 tends to return to its original dimension because itis not being cooled in the cooling chamber as the bulk material 50 is.As a result, the sacrificial puck tends to extricate itself from thereducing die as shown in FIG. 3 f. The sacrificial puck 40 can then beremoved from the deformation chamber and the process repeated after acycle time in which the bulk material 50 cools to a suitablesolidification temperature as discussed above.

Rollers

In various embodiments, working of bulk material according to theinvention is provided by passing the bulk material between sets ofrollers, while applying force to the bulk material by means of therollers to change a dimension of the material in the transversedirection. The geometry of a roller system is illustrated in a simplesystem in FIG. 7. FIG. 7 a shows a perspective view of a bulk materialin the shape of a cylinder as it is passed between rollers 100. The viewof FIG. 7 b is down the axial direction of the bulk material 50.Pressure is applied to the rollers as indicated by the arrows in FIG. 7b to change the dimension from d₁ to d₂, as illustrated in FIG. 7 c. Itis seen from FIG. 7 that one distinction between applying force byrollers and applying force by extruding is that when rollers are used,the force tends to be applied in a finite number of points around thebulk material, whereas for the extruding embodiment, force tends to beapplied all around the cross-section of the bulk material.

As is well known, the rollers 100 normally have roller or other meansfor advancing the bulk material 50, while passing the material betweenrollers that provide pinch points or restrictions. Working of thematerial is accomplished in some embodiments by passing the materialbetween a single set of rollers. If desired, additional working can beaccomplished with a single set of rollers by rolling the material backand forth between the rollers, and/or by passing the material betweenthe rollers more than once.

In various embodiments the rollers are spaced sufficiently far apart andthe velocity of the rollers is selected to provide restriction pointsfar enough apart that the compressed dimension of the material afterpassing through the first rollers recovers at least in part before beingrecompressed with a second set of rollers. This is illustrated in FIG.8. As with extrusion through a series of decreasing and increasing dies,the original dimension d₁ is reduced to d₂ and recovers to d₁′ after thefirst set of rollers. The second set of rollers compresses the dimensionto d₂′, while after the second rollers the dimension increases to d₁″,which is about the same as d1. If the distance L between the first andlast set of rollers is shorter than the total length of the materialworked, the multiple rollers showed in non-limiting fashion in FIG. 8can be set up or programmed to reverse to provide a back and forthmovement of the material between the rollers. In this way, a desiredamount of working can be applied in a mechanically convenient way. Asbefore, the dimension of the material tends to recover once the pressureapplied by the rollers is relieved.

In various embodiments, a series of rollers is provided close enoughtogether that the dimension of the material does not fully recover inthe time it takes the material to travel between the individual pairs ofrollers in the series. In this way a series of rollers is used toprovide pressure along a relatively long section of the bulk material 50as it is transported by the motion of the rollers. Such a situation isillustrated by FIG. 7 a, in a non-limiting configuration.

Whatever the spacing of the rollers, in various embodiments theindividual sets of rollers are provided with offset configurations. Forexample, a first set of rollers is provided in a first orientation asillustrated in non-limiting fashion in FIG. 7 b. A second set of rollersis provided in a second orientation, for example according to theillustration in FIG. 7 d. Thus FIG. 7 c represents deformation in afirst transverse direction to decrease the dimension to d₂, while FIG. 7d represents deformation in a second transverse direction. In thisfurther way, the material can be worked to a desired extent toaccomplish a reduction in the free radical concentration induced in thematerial by the crosslinking step.

Other configurations of rollers are possible based on the descriptionhere. The unifying aspect is that the rollers provide constrictions orpinch points that change the dimension of the material being worked inorder to induce a desired amount of material flow. It is alsoappreciated that not all the rollers in a system need be drive rollers.Rather, one of more of the rollers can be configured to provide movementto the material, while the others can be configured to providecompression only.

In various embodiments, the material is treated with rollers while beingmaintained at a compression deformable temperature, for example in aconstant temperature oven. In other embodiments, the working of thematerial by rollers occurs in ambient conditions to permit the materialto cool at the same time it is being worked. As desired, the pressure ofthe rollers applied over time can be varied, for example to slowlyreduce the pressure applied (and thus the dimension change induced inthe transverse direction) until the material has reached asolidification temperature.

In general, the rollers are configured to provide a desired pressureregime about the cross-sectional area of the bulk material. For example,a 2-roller system is illustrated in FIG. 7 a, while 3-roller systems areillustrated in FIG. 7 f, and 4-roller systems are illustrated in FIG. 7g.

The amount of working is also varied in various embodiments by providingroller means with varying speeds. The time that the bulk material spendsin a compressed state due to the action of the rollers is inverselyproportional to the speed by which the rollers move the bulk materialthrough the apparatus.

Pressure is applied by the roller systems by known mechanical means,such as without limitation gears, hydraulics, springs, and the like. Acompression ratio can be defined that is analogous to the draw ratiodefined for extrusion. Referring to FIG. 7, the compression ratio isdefined by the ratio of the original dimension d₁ to the compresseddimension d₂. In various embodiments, preferred values for therestriction ratio take on the same values as the draw ratio for theextrusion embodiment.

Compression Molding

Another way of working the material by changing a dimension in thetransverse direction is to apply pressure between plates in acompression molding fashion. The process is illustrated in anon-limiting way in FIG. 9. FIG. 9 a shows a perspective drawing of anelongate bulk material 50 between the plates 150 of a compressionmolding apparatus. Tick marks 140 show the relative orientation of thebar 50 between the plates 150. FIG. 9 b shows, in cross sectional viewlooking down the axial direction of the bulk material (show forillustration as a cylinder with circular section), the plates 150 incontact with the material before and after reduction of the transversedimension from d₁ to d₂. A compression ratio is defined as d₁/d₂, whichin various embodiments takes on preferred values such as those givenabove for draw ratio in the extrusion case. After compression, thepressure is optionally relieved (not shown) and the material is cooled.Alternatively, the material 50 is subjected to a series of compressionsto provide a desired amount of working of the bulk material. Subsequentcompressions can take place with the material in the same orientation asfor the first compression. In various embodiments, subsequentcompressions are made with the material in a different orientation withrespect to the plates. As illustrated by FIG. 9 c in a non-limitingfashion, the material is turned 90° from its first orientation, andcompression force is again applied. In various embodiments, the processof compression and pressure relief is repeated using variousorientations to achieve a desired amount of working of the bulkmaterial.

FIG. 9 shows the compressive plates oriented parallel to one another, sothat compression occurs to an equal extent all along the bulk material.The compressive method, however, is completely general, allowing forcompression between plates that are tilted relative to each other, so asto provide an unequal compression ratio along the bulk material. Afterrelieving the compression pressure, subsequent compressions can becarried out in the same or different configuration to provide thedesired amount of “working” and material flow throughout the treatedbulk material. In one embodiment, the orientation of the plates duringcontact with the bulk material is changed during contact to provide akind of rolling massage along the length of the axial direction of thebulk material. In various embodiments, at least one of the plates 150 isprovided with a curvature so that a rolling contact is maintainedbetween the bulk material and two curved plates or a flat plate and acurved plate. It is readily seen that embodiments with curved plates inarrangements like FIG. 9 have many features in common with the processof applying pressure with rollers described above.

The methods described herein—extrusion, treatment by rollers, andcompression molding, by way of non-limiting example—all involve theapplication of pressure to a bulk material in such a way that adimension in the transverse direction of the bulk material is changedfrom its original value. As a result of the pressure application,material in the bulk material flows so that the bulk material takes on adifferent shape. In many cases, pressure is applied in the transversedirection to change the dimension. In the case of the increasing die,pressure or back pressure is applied in the axial direction to providefor the material flow. However it is applied, when the pressure is thenreduced or relieved, the bulk material tends to undergo a recovery indimensions back to approximately the starting value, due to thecrosslinked nature of the bulk material on which the pressure is beingapplied. Both processes—compression and pressure relief—cause materialto flow and the bulk material to be “worked”. It is believed that thematerial flow leads to quenching of free radicals and/or thesequestering of free radicals in local environments in the bulk materialwhere they are not susceptible to reaction with adventitious oxygen,water, or other oxidizing chemicals.

In various embodiments, the methods call for sequential pressuring andrelief of pressure to produce a working or kneading of the material.Alternatively, the methods provide for a single compression and a singlepressure relief step. When the pressure is relieved and the temperatureis still above the solidification temperature (see below), the dimensionreduced by application of pressure tends to recover to approximately itsoriginal value. In some embodiments, the cross sectional area of theelongated bulk material changes (decreases or increases) in response tothe applied pressure; upon relief, the cross sectional area tends toreturn to approximately its original value. In other embodiments, thecross sectional area essentially does not change (examples are theisoareal dies, compression between rollers, and compression betweenplates) but at least one dimension in the transverse direction isdecreased; upon relief, the dimensions tend to return to their originalvalues and the shape to the original shape.

Cooling

After crosslinking, heating to a compression deformable temperature, andworking as described above, in various embodiments the polymericmaterial is cooled before further processing. Alternatively, theextruded bulk material can be directly processed by the stress reliefstep described below. In a non-limiting embodiment, the rod or otherbulk material characterized by an axial direction is cooled to asolidification temperature in a cooling chamber or other means whilepressure is maintained sufficient to keep the dimension of the extrudedbulk material below the original dimension of the crosslinked bulkmaterial. In the extrusion or other compressive force embodiments, thepressure required to maintain the dimension lower than the originaldimension may be more or less pressure than required to originallychange the shape of the polymer, such as through extrusion. As noted,the bulk material such as extruded UHMWPE is held in a cooling chamberor similar device for a sufficient time to reach a temperature at whichthe bulk material no longer has a tendency to increase in dimension uponremoval of the pressure. This temperature is designated as thesolidification temperature; for UHMWPE the solidification temperature isreached when a thermostat embedded in the cooling wall (about 1 mm fromthe inside wall surface) reads about 30° C. The solidificationtemperature is not a phase change temperature such as a melting orfreezing. It is also to be noted that a material such as UHMWPE can becooled to the solidification temperature independently of whether thematerial was heated above or below the melting point in a previousprocessing step.

In various embodiments, after extrusion or other application ofdeforming force in a direction orthogonal to the axial direction of thecrosslinked polymeric bulk material, the compressive deforming force ismaintained on the bulk material until the bulk material cools to thesolidification temperature. Such a maintenance of compressive force isconveniently provided in the reducing die embodiment illustrated inFIGS. 1 and 2. After extrusion through the reducing die 6, the bulkmaterial is held in the cooling chamber 4. In the embodiment shown inthe Figures, the cooling chamber is of such a size and shape as to holdthe extruded bulk material at a dimension or diameter d₃, which is lessthan the original dimension d₁ of the bulk material and is convenientlyabout the same as the extruded dimension d₂ in a non-limiting example.The crosslinked material has a tendency to return to its originaldimension by expanding when the temperature is above the solidificationtemperature. The expansion force of the bulk material is counteracted bythe walls of the cooling chamber, with the result that compressive forceis maintained on the bulk material while it cools. In variousembodiments, the cooling chamber is provided with cooling means such ascooling jackets or coils to remove heat from the cooling chamber and theextruded polymer bulk material.

Referring to the figures for illustration, as the polymeric extrudedbulk material cools in the cooling chamber, a temperature is reached atwhich the material no longer has a tendency to expand or revert to itsoriginal dimension d₁. At this temperature, called the solidificationtemperature, the bulk material no longer exerts pressure on the walls ofthe cooling chamber and can be removed. In preferred embodiments, thematerial is cooled to about 30° C., as measured by thermostats in thewalls of the chamber, before removal.

The temperatures of the deformation chamber and the cooling chamber canbe measured by conventional means, such as by thermocouples embeddedinto the walls of the respective chambers. For example, it has beenfound that when a thermocouple in the wall of the cooling chamberindicates a temperature of 30° C., an extruded bulk material made ofUHMWPE has reached a bulk temperature below a solidification temperatureat which the material loses it tendency to expand. The temperature asmeasured with, for example, a thermocouple embedded in the wall of thecooling chamber does not necessarily represent a bulk or equilibriumtemperature of the material in the cooling chamber. An appropriate rateof cooling may be provided in the cooling chamber by use of heatexchange fluids such as water or water glycol mixture, and the bulkmaterial held in the cooling chamber for a time and until a temperatureis reached at which it is observed that removal of the bulk materialfrom the chamber does not result in significant increase in diameter.Thus, in various embodiments, cooling to a solidification temperatureof, for example, 90° F. or 30° C. means leaving the extruded bulkmaterial in the cooling chamber until the thermocouple embedded in thewalls of the cooling chamber reads 90° F. or 30° C. As noted, it hasbeen found that such a cooling period suffices for removal of the bulkmaterial, even though the bulk equilibrium temperature of the interiorof the bulk material could be higher than the measured temperature.

In various embodiments, the extruded bulk material is held in thecooling chamber for an additional period of time, such as 10 minutes,after the embedded thermocouple reads 90° F. or 30° C. The additionalcooling period can enable the cooled material to be more easily removedfrom the cooling chamber. In one embodiment, when the thermocouplereaches a reading of 30° C., a programmable logic controller (plc)starts a timer that in turns gives a signal when the desired time haspassed. At that time an operator can remove the compression deformedcrosslinked material from the chamber, or rams or other suitable devicescan be actuated to effect removal.

Alternatively, the compression deformed and worked material is allowedto cool without applied pressure. The workpiece tends to return to theoriginal dimensions it had before the working. In various embodiments,the piece is extruded into air or into a cooling chamber of dimensionapproximately equal to that of the original workpiece. If rollers areused to compress and work the material, the roller tension can beremoved and the material allowed to cool.

Stress Relieving

As noted above, after pressure is reduced or relieved in the working ofthe bulk materials, the dimensions of the bulk materials tend to returnto approximately their original values. This is consistent with acertain amount of “shape memory” that is characteristic of crosslinkedpolymeric materials. Usually, however, the return to original dimensionsis not 100%; under various models this is ascribable to incompletereturn to an original equilibrium condition, to lack of an equilibriumcondition in the first place, or even to a series of small irreversiblechanges in structure as a result of the working. Up to a certain point,such non-equilibrium conditions can be avoided or mitigated by operatingat suitably low levels of draw ratio or compression ratio in the workingsteps described herein. But to a certain extent, some such smalldeviations from equilibrium are unavoidable. As a result, the worked andcooled material contains a certain amount of internal stress. Suchstress is subject to relief over time when held at physiologicalconditions, which could lead to dimension changes and the like of theinstalled implant. This would be of course undesirable.

For at least these reasons, the bulk material is preferably stressrelieved following working of the material in the transverse directionand optional cooling to a solidification temperature, and prior to beingmachined into a bearing component for use in a medical implant such asan artificial joint. In one embodiment, stress relieving is carried outby heating to a stress relief temperature, preferably below the meltingpoint of the polymeric bulk material. If the cooling in the previousstep is carried out while maintaining deformation force, the bulkmaterial on stress relieving tends to expand and return to a dimensionclose to its original dimension. In the non-limiting example of anextruded rod, as the bulk material is heated, the diameter d₃ of the rodtends to increase to a diameter approaching d₁ of the original bulkmaterial. In various non-limiting embodiments, it has been observed thatthe bulk material retains about 90-95% of its original dimension uponstress relieving or stress relief heating.

Stress relief is carried out in a variety of ways. In variousembodiments, the bulk material is heated in an oven, is heated byinfrared radiation, is subject to microwave radiation, or is treatedwith ultrasonic energy. In various aspects, the methods tend to increasethe temperature of the bulk material and provide a thermal defect.However, it is believed that some of the methods, for exampleultrasound, provide various energy other than thermal energy to providefor the stress relieving.

For thermal-based stress relieving, the stress relief process tends torun faster and more efficiently at higher temperatures. Accordingly,stress relief temperatures close to but less than the meltingtemperature are preferred, for example from the melting point to themelting point minus 30 or 40° C. For UHMWPE, preferred stress relieftemperatures include in the range of about 100° C. to about 135° C.,110° C. to about 135° C., 120° C. to 135° C., and preferably 125° C. toabout 135° C.

Stress relieving is carried out for a time to complete the stress reliefprocess. In various embodiments, suitable times range from a few minutesto a few hours. Non-limiting examples include 1 to 12 hours, 2 to 10hours, and 2 to 6 hours in an oven or other suitable means formaintaining a stress relief temperature. Although the stress relievingcan be carried out in a vacuum, in an inert atmosphere, or in a packagedesigned to exclude an atmosphere, it is preferably carried out in anair atmosphere.

Under some conditions, the solidified extruded bulk form exhibits atendency to bend or otherwise deviate from a preferred straight orlinear orientation during the heating or other treatment associated withstress relieving. To counter this tendency, in one embodiment, the bulkmaterial is held in a mechanical device that functions to keep the bulkmaterial straight (measured on the axial direction) during the stressrelieving step. In a non-limiting example, the bulk material is placedinto V-channels to keep it straight. For example, several V-channels areequally spaced from each other and are part of the same physicalstructure. The several V-channels may, for example, be welded to thestructure at equal spacings. The extruded bars are positioned on abottom set of V-channels and then another set of V-channels is set ontop of the extruded bars to rest on top of the bars. These channels helpto keep the bars straight during stress relieving.

In various embodiments, the product of the crosslinking, heating,compressing (working), cooling, and stress relieving steps is a bulkmaterial having dimensions approximately equal to the original bulkmaterial before crosslinking. As a result of the steps taken on the bulkmaterial, the bulk material exhibits high tensile strength in the axialdirection, a low but detectable level of free radical concentration, anda high degree of resistance to oxidation.

The process described can be followed with regard to the dimensions ofthe crosslinked polymer at various stages of the process. In variousembodiments, a bulk material having an original dimension or diameter ofd₁ is crosslinked and heated to a compression deformation temperature.The crosslinked heated material is then compressed to a dimension ordiameter d₂ which is less than d₁. In an optional step, the material isthen held while cooling at a diameter d₃ that may be the same as d₂, butin any case is less than the original dimension or diameter d₁. Aftercooling, stress relieving returns the bulk material to a diameter d₄which is greater than d₃ and in some embodiments is approximately equalto the original dimension or diameter d₁. For example, if the originalbulk material is a 3″×14″ cylinder of UHMWPE, the treated preformresulting from the steps above preferably typically has a diameter ofabout 2.7 to 3 inches.

Following the treatment steps described above, the bulk materialcharacterized by an axial direction is machined according to knownmethods to provide bearing components for implants. In the case of acylindrical treated bulk material perform, it is preferred first to turnthe outer diameter of the cylinder to remove any oxidized outer layersand to provide a straight and round cylinder for further processing. Ina preferred embodiment, the cylinder is then cut into billets along theaxial direction, and each billet is machined into a suitable bearingcomponent. Preferably, the bearing components are machined from thebillets in such a way that the in vivo load bearing axis of the bearingcomponent corresponds to the axial direction of the bulk preform fromwhich it is machined. Machining this way takes advantage of theincreased tensile strength and other physical properties in the axialdirection of the preform.

For example, in bearing components for joint replacements, the stressesat the bearing surface are typically multiaxial, and the magnitude ofthe stresses further depends on the conformity of the joint. For hipapplications, the polar axis of the cup is aligned with the longitudinalaxis of the extruded rod, corresponding to the axial direction. The wallof the cup, at the equator and rim, is parallel to the long axis of therod, and will benefit from the enhanced strength in this directionduring eccentric and rim loading scenarios.

Oxidative Resistance

It has been found that UHMWPE preforms, and bearing components madeaccording to the invention have a high level of oxidative resistance,even though free radicals can be detected in the bulk material. Tomeasure and quantify oxidative resistance of polymeric materials, it iscommon in the art to determine an oxidation index by infrared methodssuch as those based on ASTM F 2102-01. In the ASTM method, an oxidationpeak area is integrated below the carbonyl peak between 1650 cm⁻¹ and1850 cm⁻¹. The oxidation peak area is then normalized using theintegrated area below the methane stretch between 1330 cm⁻¹ and 1396cm⁻¹. Oxidation index is calculated by dividing the oxidation peak areaby the normalization peak area. The normalization peak area accounts forvariations due to the thickness of the sample and the like. Oxidativestability can then be expressed by a change in oxidation index uponaccelerated aging. Alternatively, stability can be expressed as thevalue of oxidation attained after a certain exposure, since theoxidation index at the beginning of exposure is close to zero. Invarious embodiments, the oxidation index of crosslinked polymers of theinvention changes by less than 0.5 after exposure at 70° C. to fiveatmospheres oxygen for four days. In preferred embodiments, theoxidation index shows a change of 0.2 or less, or shows essentially nochange upon exposure to five atmospheres oxygen for four days. In anon-limiting example, the oxidation index reaches a value no higher than1.0, preferably no higher than about 0.5, after two weeks of exposure to5 atm oxygen at 70° C. In a preferred embodiment, the oxidation indexattains a value no higher than 0.2 after two or after four weeksexposure at 70° to 5 atm oxygen, and preferably no higher than 0.1. In aparticularly preferred embodiment, the specimen shows essentially nooxidation in the infrared spectrum (i.e. no development of carbonylbands) during a two week or four week exposure. In interpreting theoxidative stability of UHMWPE prepared by these methods, it is to bekept in mind that the background noise or starting value in theoxidation index determination is sometimes on the order of 0.1 or 0.2,which may reflect background noise or a slight amount of oxidation inthe starting material.

Oxidation stability such as discussed above is achieved in variousembodiments despite the presence of a detectable level of free radicalsin the crosslinked polymeric material. In various embodiments, the freeradical concentration is above the ESR detection limit of about0.06×10¹⁵ spins/g and is less than that in a gamma sterilized UHMWPEthat is not subject to any subsequent heat treatment (aftersterilization) to reduce the free radical concentration. In variousembodiments, the free radical concentration is less that 3×10¹⁵,preferably less 1.5×10¹⁵, and more preferably less than 1.0×10¹⁵spins/g. In various embodiments, the oxidation stability is comparableto that of melt processed UHMWPE, even if according to the invention theUHMWPE is processed only below the melting point.

Although the invention is not to be limited by theory, the free radicalsin the deformation processed UHMWPE described above may be highlystabilized and inherently resistant to oxidative degradation.Alternatively or in addition, they may be trapped within crystallineregions of the bulk material and as a consequence may be unavailable toparticipate in the oxidation process. Because of the oxidation stabilityof the material, in various embodiments it is justifiable to employ gaspermeable packaging and gas plasma sterilization for the processedradiation UHMWPE. This has the advantage of avoiding gammasterilization, which would tend to increase the free radicalconcentration and lead to lower oxidation stability.

In various embodiments, the solid state deformation process providespolymers that are characterized by a crystal and molecular orientation.By molecular orientation is meant that polymer chains are orientedperpendicular to the direction of compression. By crystallineorientation it is meant that crystal planes in polyethylene, such as the200 plane and the 110 plane are oriented to the direction parallel tothe compression plane. In this way the crystal planes are oriented. Thepresence of the orientations can be shown by means of birefringentmeasurements, infrared spectra, and x-ray diffraction.

The plane of compression for articles compressed in a radial directionis understood to be a surface surrounding and parallel to the radialsurface of the bulk material that is processed according to theinvention. In the non-limiting example of a cylindrical rod, a sequenceof circular cross sections along the axial direction defines a radialsurface and a compression plane perpendicular to that surface. Inresponse to compression around the radial plane, polymer chains orientthemselves perpendicular to the direction of compression. This has theeffect in a cylinder of providing molecular orientation generallyparallel to the radial plane. It is believed that with this molecularand crystal orientation contributes to the enhancement of mechanicalproperties, and to anisotropy in the mechanical properties with respectto the axial and transverse (or radial) directions.

In various embodiments, crosslinked UHMWPE are provided that exhibit ahigh level of tensile strength in at least one direction.Advantageously, bearing components and implants are provided that takeadvantage of the increased strength of the bearing material. Forexample, in crosslinked UHMWPE, it is possible to achieve a tensilestrength at break of at least 50 MPa, preferably at least 55 MPa, andmore preferably at least 60 MPa. In various embodiments, materials areprovided with a tensile strength at break in the range of 50-100 MPa,55-100 MPa, 60-100 MPa, 50-90 MPa, 50-80 MPa, 50-70 MPa, 55-90 MPa,55-80 MPa, 55-70 MPa, 60-90 MPa, 60-80 MPa, and 60-70 MPa. In anon-limiting embodiment the tensile strength of a UHMWPE preparedaccording the invention is about 64 MPa in the axial direction.

Embodiments of the present invention are further illustrated through thefollowing non-limiting examples.

EXAMPLES Comparative Example

Isostatically molded UHMWPE bar stock (Ticona, Inc., Bishop, Tex.) ispackaged in an argon environment and gamma sterilized to a dose of 25 to40 kGy

Example 1

Radiation crosslinked, deformation processed UHMWPE is produced usingthe following steps:

1. Radiation crosslinking. Isostatically molded UHMWPE rods ofdimensions 3″×14″ (GUR resin from Ticona, Inc., Bishop, Tex. fullyconsolidated according to the isostatic pressure steps described in U.S.Pat. No. 5,688,453) are vacuum packed in a foilized bag and gammaradiation crosslinked with a nominal dose of 50 kGy.

2. Preheating. Prior to deformation processing, the rod is removed fromthe foilized bag and raised to 133° C. for 4 to 12 hours in an oven.

3. Solid state, hydrostatic extrusion. The heated rod is then removedfrom the oven and placed in the holding chamber of a press. Thetemperature of the holding chamber is 130° C.±5° C. The bar is then ramextruded using a sacrificial puck made of crosslinked UHMWPE through acircular die, into a cooling chamber with a diametral compression ratioof 1.5 (diameter of 3″ down to 2″).

4. Cooling and solidification. The cooling chamber is sized so as tomaintain the extruded rod in a deformed state. The walls of the coolingchamber are water-cooled. When thermocouples embedded in the wall (about1 mm from the inside wall) read 30° C., the solidified rod is removed,optionally after an additional cooling period of ten minutes, in anon-limiting example. If desired, a second bar is ram extruded to ejectthe cooled bar from the cooling chamber, once the temperature reachesabout 30° C.

5. Stress relief, annealing. The deformed rod is then heated at 133±2°C. for 5 hours. The annealing also improves dimensional stability in thematerial. The rod is then slowly cooled to room temperature. Theextruded rod retains about 90-95% of its initial diameter after thestress relief step.

6. Gas plasma sterilization. After cooling, a liner or other bearingmaterial is machined and the machined part is non-irradiativelysterilized (e.g., with ethylene oxide or gas plasma)

Specimen Preparation and Orientation

For compression tests and accelerated aging, right rectangular prismspecimens are evaluated. The specimens measure 12.7 mm by 12.7 mm by25.4 mm (0.50 in. by 0.50 in. by 1.00 in.) They are machined from therod stock parallel (the axial direction) or perpendicular (thetransverse direction) to the long axis.

For tensile tests, dumbbell-shaped tensile specimens consistent with theType IV and V specimen description provided in ASTM D638-02a are tested.Specimens are 3.2±0.1 mm thick. Specimens are oriented parallel orperpendicular to the long axis, reflecting the axial and transversedirections, respectively).

Physical and Mechanical Properties

Tensile strength at break is determined according to ASTM 638-02a.

The concentration of free radicals in the UHMWPE materials ischaracterized using an ESR spectrometer (Bruker EMX), as describedpreviously in Jahan et al., J. Biomedical Materials Research, 1991; Vol.25, pp 1005-1017. The spectrometer operates at 9.8 GHz (X Band)microwave frequency and 100 kHz modulation/detection frequency, and isfitted with a high sensitivity resonator cavity. For a good spectralresolution and/or signal-to-noise ratio, modulation amplitude is variedbetween 0.5 and 5.0 Gauss, and microwave power between 0.5 and 2.0 mW.

Accelerated Aging

Specimens are aged in 5 atmospheres of oxygen in accordance with ASTM F2003-00. Some specimens are aged for two weeks according to thisstandard, and others are aged for four weeks. Aging is performed in astainless steel pressure vessel. The specimens are chosen and orientedsuch that the tested axis is vertical. Thus, the top and bottom facesare perpendicular to the test axis. The top face is labeled for lateridentification. The vessels are then filled with oxygen and purged fivetimes to ensure the purity of the aging environment. The prisms rest ona flat surface inside the pressure vessel; thus each prism's bottom faceis not exposed to oxygen, but each of its other faces are exposed tooxygen throughout the aging period.

The vessel is placed in the oven at room temperature (24±2° C.), and theoven was heated to the aging temperature of 70.0±0.1° C. at a rate of0.1° C./min.

FTIR Analysis

Materials are evaluated before and after accelerated aging by Fouriertransform infrared spectroscopy (FTIR) in transmission (Excalibur seriesFTS3000 with a UMA-500 microscope attachment; Bio-Rad Laboratories,Hercules, Calif.). FTIR profiling is conducted perpendicular to thetransverse direction.

Oxidation index measurement and calculations are based on ASTM F2102-01. Oxidation peak area is the integrated area below the carbonylpeak between 1650 and 1850 cm⁻¹. The normalization peak area is theintegrated area below the methylene stretch between 1330 and 1396 cm⁻¹.Oxidation index is calculated by dividing the oxidation peak area by thenormalization peak area.

Data for the Comparative Example and Example 1 are given in the Table

Comparative Example 1, Example 1, Example Example 1 axial transverseTensile Strength 46.8 ± 2.0  64.7 ± 4.5 46.1 ± 3.5 at Break [MPa] Freeradical 3.82 × 10¹⁵ 0.22 × concentration, 10¹⁵ spins/g Oxidation index0.2 <0.1 before aging (at surface) Oxidation index 1.2 <0.1 after aging(at surface)

Example 2 Isoareal Restriction

A bulk UHMWPE in the form of a 3″×14″ cylinder is subject to 5 Mrad ofgamma irradiation to crosslink it. The crosslinked bar is preheated to130° C. for 4 hours. The bar is then extruded through a constant areaelliptical mold tool. The cross-sectional area is maintained about 7 sq.in., which is the cross-sectional diameter of the bar. Inside theelliptical mold tool, the shape starts at a 3 inch diameter circle andchanges into an ellipse (1.5 in. minor axis×6 in. major axis) at aposition 0.75 inches from the start. The tool changes back to 3 inchdiameter circle at position 1.5 inches from the start. Then the toolchanges to an ellipse (1.5 inch minor×6 inch major axis) at position2.25 inches from the start. The second ellipse is oriented 90° to thefirst ellipse. Then the tool changes back to a 3 inch diameter circle ata position 3 inches from the start. The bulk material is extruded fromthe elliptical material mold tool where it is cooled. During cooling,the bars change shape from a 3 inch diameter to a rounded offrectangular shape. A stress relief cycle is carried out by heating to130° C. for 4 hours. After heating for 4 hours, the bar is slowlycooled. After stress relief, the bars maintain a rounded off rectangularshape.

The tensile strength of the bar in the axial direction is about 8000 psi(about 55.2 MPa) while the impact strength in the transverse directionis about 70-75 kJ/m². The free radical concentration measured by epr isabout 2.6×10¹⁴ spins/g (or 0.26×10¹⁵ spins/g).

The examples and other embodiments described herein are exemplary andnot intended to be limiting in describing the full scope of compositionsand methods of this invention. Equivalent changes, modifications andvariations of specific embodiments, materials, compositions and methodsmay be made with substantially similar results.

1. A method for reducing the free radical concentration in irradiatedcrosslinked bulk polymer, the polymer being in the form of a bulkmaterial elongated in an axial direction, the method comprising: heatingthe crosslinked bulk material to a compression deformable temperature;applying a force to deform the heated bulk material in a directionorthogonal to the axial direction by extruding the heated bulk materialthrough an isoareal die; and cooling the polymer to a solidificationtemperature.
 2. A method according to claim 1, wherein the bulk polymeris irradiated with gamma-irradiation.
 3. A method according to claim 1,wherein the bulk polymer comprises UHMWPE.
 4. A method according toclaim 3, wherein the bulk polymer comprises a bar having a circularcross-section.
 5. A method according to claim 4, wherein the circularcross-section has a diameter of from 2-4 inches.
 6. A method accordingto claim 5, wherein the circular cross-section has a diameter of about 3inches.
 7. A method according to claim 1, comprising multiple extrusionthrough a single die and/or extrusion through a plurality of dies.
 8. Amethod according to claim 1, wherein the compression deformabletemperature is less than the crystalline melting point of the polymer.9. A method of processing a crosslinked polymeric material, the materialin a bulk form characterized by an axial direction, and furthercharacterized by a cross section having an area A in a transversesection perpendicular to the axial direction, the method comprisingheating the polymeric material to a compression deformable temperature;deforming the heated material by extruding it through a die that changesthe dimensions of the cross-section, but that leaves the cross-sectionalarea A essentially unchanged; and cooling the extruded material.
 10. Amethod according to claim 9, wherein the extruded material ischaracterized by a free radical concentration less than that of thecrosslinked material before extrusion.
 11. A method according to claim9, wherein the compression deformable temperature is below the meltingpoint.
 12. A method according to claim 9, wherein the polymeric materialis in the form of a cylinder.
 13. A method according to claim 12,wherein the cylinder has a circular cross-section perpendicular to theaxial direction of the cylinder.
 14. A method according to claim 9,wherein the polymeric material comprises UHMWPE.
 15. A method accordingto claim 14, wherein the UHMWPE is crosslinked by gamma-irradiation at adose of 1 to 100 MRad.
 16. A method according to claim 14, wherein thepolymeric material is in the form of a cylinder.
 17. A method accordingto claim 16, wherein the cylinder has a circular cross- sectionperpendicular to the axial direction of the cylinder.
 18. A methodaccording to claim 14, further comprising machining a medical implantbearing component from the extruded material, wherein the load bearingaxis of the implant is substantially coincident with the axial directionof the polymeric material.
 19. A method of making a medical implantbearing component, comprising: crosslinking a UHMWPE bulk material toproduce a free radical concentration greater than 0.06×10¹⁵ spins pergram; heating the crosslinked UHMWPE to a compression deformabletemperature; extruding the heated UHMWPE, in the form of an elongatebulk material comprising a main axis defining an axial direction, andfurther characterized by a cross-section perpendicular to the axialdirection, the extruding being though a die that has a shape differentfrom that of the cross-section, but having an area essentially the sameas the area of the cross-section; and further processing the extrudedUHMWPE to make the bearing component.
 20. A method according to claim19, comprising exposing the UHMWPE bulk material to gamma-irradiation ata dose of about 0.01 to about 100 MRad.
 21. A method according to claim19, wherein the compression deformable temperature is less than themelting point.
 22. A method according to claim 19, wherein thecompression deformable temperature is above 80° C. and less than thecrystalline melting point of the UHMWPE.
 23. A method according to claim19, wherein the cross-section of the UHMWPE bulk material is essentiallycircular.
 24. A method according to claim 19, wherein the shape of thedie is a smooth convex shape.
 25. A method according to claim 19,wherein the shape of the die is elliptical.